The configuration of a typical Bluetooth receiver is introduced in FIG. 1. Referring to FIG. 1, the Bluetooth receiver includes a front-end circuit 110 configured to obtain a baseband signal by demodulating a received signal, a filter 120 configured to selectively pass a desired channel therethrough and remove an undesired channel, and a recovery circuit 130 configured to convert the received baseband signal in the frequency domain into time domain symbols.
A Bluetooth transmitter generates a carrier frequency-based modulated signal by modulating a baseband signal. A Bluetooth Smart transmitter uses a Gaussian frequency shift keying modulation method that has a modulation index h ranging from 0.45 to 0.55. The frequency shift keying method refers to a method of transmitting digital information through the variation of the discrete frequency of a carrier.
FIG. 11 is a diagram showing the frequency characteristic of a modulated signal that is transmitted by a Bluetooth transmitter in an ideal environment in which noise and frequency offset are not present. Referring to FIG. 11, there are shown the minimum and maximum frequency shifts of a signal having a symbol transmission speed Fs of 1 Msps and a modulation index h of 0.5 that is transmitted via a carrier frequency Fc in the 2.4 GHz band. When the symbol transmission speed Fs is 1 Msps, the signal of the bit value “1” corresponding to the symbol “+1” has a frequency shift F+ of +250 kHz (a frequency shift in a positive (+) direction) from a center frequency Fc, the signal of the bit value “0” corresponding to the symbol “−1” has a frequency shift F− of −250 kHz (a frequency shift in a negative (−) direction) from the center frequency Fc.
Referring back to FIG. 1, the front-end circuit 110 of the Bluetooth Smart receiver obtains a frequency-demodulated waveform in a baseband by using an analog or digital frequency demodulator, and estimates transmission bit information by deciding signs at symbol intervals.
Since a signal is received in the state in which the quality thereof has been degraded due to signal magnitude offset, carrier offset, timing offset, etc. attributable to mismatch between a transmitter and the receiver, the receiver must be prepared for errorless bit demodulation by implementing a recoverer for corresponding offset. In particular, when carrier offset is generated, a frequency-demodulated waveform in a baseband exhibits the state in which the average value of frequency shifts is not zero and is biased by a constant value corresponding to the magnitude of the carrier offset.
FIG. 2 is a diagram showing a typical packet of Bluetooth Smart. Referring to FIG. 2, the packet of Bluetooth Smart includes a preamble interval 210, an access address interval 220, a protocol data unit (PDU) interval 230, and a CRC interval 240. Since a Bluetooth receiver must identify an address during the access address interval 220 and must identify and process data during the PDU interval 230, preparation for the identification of the address and the data must be completed during the preamble interval 210. Accordingly, there is a time limitation in that operations, such as automatic gain control, frequency offset compensation, timing compensation, etc., must be performed within a preamble interval of Bluetooth or Bluetooth Smart in the front-end circuit 110 of the Bluetooth receiver.
For a receiver to estimate offset, a previously agreed upon pilot signal is required between a transmitter and the receiver. According to the Bluetooth Smart standard, a bit stream corresponding to the start of a packet is transmitted in the preamble interval 210. The bit stream of the preamble interval 210 is determined by the first transmission bit of the access address interval 220. When the first transmission bit of the access address interval 220 is “1,” the bit stream value “01010101b” of the preamble interval 210 is transmitted. When the first transmission bit of the access address interval 220 “0,” the bit stream value “10101010b” of the preamble interval 210 is transmitted. Since the frequency-demodulated waveform of the preamble interval 210 has a sine wave-like form in which negative (−) and positive (+) frequency shifts are repeated, it has the characteristic of a pilot signal appropriate for the estimation of carrier offset using a “minimum-maximum average value” scheme.
An example of a preceding technology for compensating for the frequency offset of a received signal in a Bluetooth receiver is disclosed in U.S. Pat. No. 6,642,797 entitled “Normalization Methods for Automatic Frequency Compensation in Bluetooth Applications.”
FIG. 3 is a circuit diagram showing a circuit for compensating for frequency offset according to the preceding technology. Referring to FIG. 3, the circuit for compensating for frequency offset includes an analog-to-digital converter 310, a low-frequency pass filter 320, a frequency demodulator 330, a digital peak detector 340, and an offset normalizer 350. The preceding technology is a technology for correcting or compensating for frequency offset during the preamble interval 210, having a setting similar to that of FIG. 2, for each signal packet.
The preceding technology is configured such that the digital peak detector 340 detects an average frequency by means of a minimum-maximum average value by detecting the maximum positive and negative peaks of a frequency component and estimates offset by calculating the difference between the average frequency and a preset carrier frequency, and the offset normalizer 350 compensates for the difference.
Since the preamble 210 is an agreed upon bit pattern in which a negative (−) frequency shift and a positive (+) frequency shift appear symmetrically, all offset other than zero is made to pertain to frequency offset by applying a “minimum-maximum” average value. However, since a bit pattern has a random characteristic in the user data intervals 220 and 230, a negative (−) frequency shift and a positive (+) frequency shift do not appear symmetrically and unspecific offset attributable to asymmetry is mixed with a frequency offset component, and thus a disadvantage arises in that the validity of a frequency offset estimation method using an instantaneously obtained “minimum-maximum” average value for the preamble interval 210 is poor.
A preceding technology using a method of predicting the tendency of changes in offset and performing feed-forwarding so that the offset estimated in the preamble interval 210 can be used in the user data intervals (the access address interval 220 and the protocol data unit interval 230) is disclosed in U.S. Pat. No. 8,411,797 entitled “Frequency Offset Compensation in a Digital Frequency Shift Keying Receiver.”
The second preceding technology employs a statistical “minimum-maximum” average value scheme in order to overcome the disadvantage of the method of estimating frequency offset using an instantaneous “minimum-maximum” average value in the random data intervals 220 and 230. That is, a statistical characteristic is used in which an instantaneous value has low accuracy due to offset attributable to data but maximum positive and negative peaks converge to a symmetrically uniform value when observed over a long period of time. The maximum positive and negative peaks are obtained using a moving average or sliding average method, the randomness effect of data is removed, and the tendency of minute changes in frequency offset is tracked using the intermediate value of the two peaks. Although frequency offset may be estimated in the access address interval 220 and the protocol data unit interval 230 by using the above method, the method is Useful in a desirable received signal region having a value equal to or higher than −90 dBm in which the magnitude of white noise is relatively low because there is a risk that an estimated error may be amplified when white noise is added to an environment where the randomness of data is present.
In a current situation in which a demand for a high-sensitivity receiver supporting a value equal to or lower than −90 dBm is increasing, it is difficult to perform sufficient offset compensation on a Bluetooth Smart signal by using the conventional preceding technologies. Therefore, there is an increasing need for a means that is capable of dealing with this situation.